Menthol (1), the principal constituent of wild mint (Mentha arvensis), is among the most important aroma compounds in the fragrance and flavors industry and may take the form of four diastereomers. The oxidized diastereomers of menthol are referred to as menthone (2) and isomenthone (3), respectively. In the case of the diastereomers of menthone, the two alkyl substituents are in trans position; in the case of isomenthone, the substituents are in cis configuration.
On the basis of its odor and flavor profile, which is reminiscent of peppermint, menthone is employed in numerous formulations, for oral care and chewing gum applications, for example.

Menthone is customarily obtained by oxidation starting from menthol. Oxidizing agents used in this process are sodium chromate/sulfuric acid (Spec. Chem. 1987, 7, 193; Acta Chem. Scand. B 1979, 33, 148), sodium hypochlorite/acetic acid (J. Org. Chem. 1980, 45, 2030), ozone/ethyl acetate (JP 82180463) or pyridinium chlorochromate/silica gel (Tetrahedron 1979, 35, 1789). All in all, the oxidative methods for menthone preparation cannot be considered to be satisfactory, in view of the use of reagents which are of concern in relation to occupational hygiene and environmental compatibility.
DE 4236111 A1 describes a process for preparing menthone from menthol in a fixed-bed reactor, using a heterogeneous, copper-based dehydrogenation catalyst.
W. Treibs et al. (Chem. Ber. 1927, 60, 2335) describe the reaction of isopulegol (4) to menthone (2) in the presence of a catalyst which is obtained from copper acetate by precipitation with NaOH.

At a reaction temperature of 280° C., after the starting material has been passed over the catalyst, a mixture of menthone, isomenthone, and thymol can be isolated. The reaction regime disclosed therein is considered disadvantageous in view primarily of the awkward preparation of the catalyst and the high temperatures, which promote the formation of by-products.
Another means of preparing menthone, in this case starting from citronellol (5), is described in U.S. Pat. No. 4,134,919.

Here, over the course of four hours, at temperatures of 150-260° C. and under continuous hydrogen pressure, citronellol is reacted to give a menthone/isomenthone mixture. Catalysts used are copper-based compounds, e.g., Cu/Cr, Cu/Al, or Cu/Zn. Isomenthone and menthone can also be obtained in enantiomerically enriched form. In that case enantiomeric excesses of not more than 80% are attained.
Citronellal (6) as well can be used as a starting material for preparing menthone. Forti et al. (Synthesis 2001, 1, 52) describe the cyclization of citronellal in the presence of calcium phyllosilicates, aluminum nitrate, and iron nitrate, and also of a solution of NaOH in dichloroethane. The product mixture contains menthone and isomenthone in a ratio of 69/31.

In summary it is observed that the processes known from the prior art do allow the synthesis of menthone, even in enantiomerically enriched form, but entail the use of reagents which are harmful to health or the environment. Moreover, the processes disclosed only afford enantiomeric excesses of 80% at most. Moreover, the conduct of the reaction primarily in liquid reaction media, and the use of catalysts which are difficult to obtain, represent substantial disadvantages.
It is an object of the invention, therefore, to provide an Improved process for the catalyzed preparation of menthone, in which menthone in particular is obtained with an increased fraction of one enantiomer, more particularly in enantiomerically pure form.